Variation in Hieracium subgen. Pilosella (Asteraceae):: What do we know about its sources?

The present paper reviews mechanisms producing complicated patterns of variation within Hieracium subgen. Pilosella. The taxonomic complexity of this subgenus is due to highly variable basic species and intermediate (hybridogenous) species. The most important sources of variation are polyproidy, hybridization and (mostly) facultative apomixis of the aposporous type. The combination of hybridization, apomixis and clonal growth leads to the maintenance of various hybrids having originated from backcrossing and hybridization among more than two species, which is possible because of the fertile pollen of apomictic hybrids. Ever since Mendel's experiments, some of F-1 hybrids have been found to be highly variable, probably reflecting the high heterozygosity of some of the basic species. Variable progeny can also result from unreduced gametes, or the rare parthenogenetic development of reduced gametes. While these processes were detected in experiments, their role within field populations remains unknown. However, multiple origins of intermediate species, and introgression within basic species are highly likely to result in high levels of variation. While few population level studies have been undertaken in Europe, several such studies have been carried out on adventive populations in New Zealand, and these show a different pattern. Aneuploid plants, rare in Europe, are common in New Zealand, and there is frequently more than one ploidy level within a population.